fresh 727 problem

The drums looked good used steel rings, the reverse problem has only been a issue since I rebuilt it ,I have had the trans a long time ,what is the benefit of a billit servo ,thinking I may put it back together ,install a temp gauge and figure out how to perform hydraulic pressure testing on it ,and make sure it is not getting hot ,I did install a b&m cooler with fan in the rear of the car during the rebuild , hopefully it is not a case or bushing problem.